Magnetic induction type electron accelerator



Jan. 7, 1958 R; WIDERUE 2,819,393

MAGNETIC mnuc'rrou TYPE ELECTRON ACCELERATOR Filed April 1, 1953 2Sheets-Sheet 1 l f i a A INVENTOR BY M JWNXQPJQW ATTORNEYS Jan. 7, 1958R. WIDERGE 2,819,393

' MAGNETIC moucnou TYPE ELECTRON ACCELERATOR Filed April 1. 195a 2Sheets-Sheet 2 V INVENTOR BY JJI MM MQM W ATTORNEYS United States Patent"ice MAGNETIC INDUCTION TYPE ELECTRON ACCELERATOR Rolf Wideriie,Ennetbaden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri &Cie, Baden, Switzerland, a joint stock company Application April 1,1953, Serial No. 346,123

Claims priority, application Switzerland April 3, 1952 Claims. (Cl.250-27) In an induction accelerator, the electrons moving in theevacuated acceleration tube are led away from the circular path traveledduring the acceleration by an enlargement or reduction of theequilibrium circle (expansion or contraction) always at the end of theacceleration period. By this means the electrons can be directed forexample toward an anticathode disposed in the interior of theacceleration tube near the circular path, where they produce X-rays, orthey can be led through the tube wall to the outside and leave theaccelerator for other uses. Expansion and contraction are usuallyeffected by variation of the ratio between the flux accelerating theelectrons and the control flux maintaining the electrons in their orbitand occurring between the circular so-called control poles. These twofluxes can be produced by a single exciter winding fed from thealternating current network. To vary the ratio, use can be made ofsaturation phenomena in the iron parts of the accelerator; or specialcoils may be provided which when they carry current vary at least one ofsaid fluxes. For example, a connection for the generation of expansionimpulses is known from Swiss Patent No. 260,195 which contains a coilthat embraces the acceleration flux and in which, through the dischargeof a condenser, current impulses are produced which effect anenlargement of the electron path. The recharging of the condenser occursby the same coil as a result of the voltage induced in it by theacceleration flux. This connection is suitable also for accelerators inwhich electrons are accelerated alternately in opposite directions inconsecutive half-periods of the alternating current supply network(duo-radiation accelerator), because the direction of the currentimpulses alternates also.

It may now be desirable to employ for the utilization of the energy ofthe accelerated electrons selectively an expansion or a contraction,either so as to produce selectively electron rays or X-rays, or in orderselectively to bombard one of two anticathodes, one of which beingarranged within and the other without the acceleration circle inside theacceleration tube.

The present invention relates to a device for the selective use ofexpansion or contraction, requiring a minimum of means. The device ofthe invention is characterized by a coil which embraces the accelerationflux, two thyratrons connected in parallel with opposite polarity, meansfor the alternate supply of positive voltage impulses to the controlgrid of the thyratrons each time the network voltage passes throughzero, a condenser connected in series with the two thyratrons, and areversing switch which connects the assembly formed by the condenser andthe thyratrons, in one position of the switch, with the coil, and in theother position, with the series connection of the coil and exciterwinding, this series connection being poled in such a way that thevoltages connected at the coil and exciter winding are opposed.

The embodiment of the invention to be described is illustrated in theaccompanying drawings in which Fig. 1 is an electrical schematic diagramshowing the essential components and their circuit relationship. Figs.2-4 are graphs picturizing operation of the circuit in a sequentialmanner.

With reference now to the drawings the exciter winding 1 for theaccelerator is fed from the alternating current network indicated bysymbol with the voltage U connected in parallel thereto is a condenser 5for the improvement of the output factor. The coil 2 serves for thevariation of the ratio between acceleration flux and control flux. Itembraces the acceleration flux alone, and hence is disposed spatiallywithin the control poles of the accelerator. In this coil an alternatingvoltage U; is induced by the acceleration flux. A condenser 4 isconnected in series with two thyratrons 6 and 7 connected in parallelwith opposite polarity i. e. in front-to-back relation. The arrangement8 includes means which alternately supply to the control grids of thetwo thyratrons positive impulses, and impulses which are in locked phaserelation with the network voltage. These impulses begin approximatelywith each passage of the network voltage through zero and last somewhatlonger than one quarterperiod. In its one position (position e), thereversing switch 9 connects the series connection of the condenser 4 andthe thyratrons with the ends of coil 2, in its other position (positionk) with the ends of the series connection of coil 2 and exciterwinding 1. The latter parts are connected in opposing directions, thatis, the voltages U and U connected to them have opposite directions. Forthis purpose, the end a of coil 2 is connected with the co-phased end I:of the exciter winding 1. Simultaneously with the reversal effected bythe reversing switch 9, the phases of the positive voltage impulsessupplied to the control grids of the thyratrons are changed by 180 inrelation to the network voltage by the reversing switch 10. The mode ofoperation of the device of the invention will be described withreference to Figs. 24, which show operation of the circuit inchronological sequence.

Curve U of Fig. 2 represents the course of the feeding network voltage,referred to point b of Fig. 1. Let it be noted that the electrons reachtheir maximum energy when the acceleration flux is maximum, that is,when the network voltage passes through zero. In that instant theelectrons must be removed from the circular path by a current impulseflowing in coil 2. The arrows marked 1 I and 11, respectively, on thetime axis of Fig. 2 indicate the temporal position of the accelerationprocesses in a duo-radiation accelerator; the removal of the electronsfrom the circular path always occurs at the tips of the arrows.

Curve U of Fig. 2 represents the course of the alternating voltageinduced by the acceleration flux in coil 2, referred to point e ofFig. 1. For reasons which will be stated later, the amplitude of thisalternating voltage is half the amplitude of the network voltage.

Fig. 3 shows the operations occurring during expansion. It ispre-supposed, therefore, that reversing switch 9 (Fig. 1) is in positione. Curve U again represents the course of the alternating voltageinduced in coil 2 and curve U the course of the voltage connected atcondenser 4, also referred to point c of Pig. 1. Shortly before the endof the first entered half-period, the condenser 4 is charged on thepositive peak of voltage U In that instant of the expansion, that is,near the passage of the network voltage through zero, the discharge ofthe condenser 4 via coil 2 is made possible again by ignition of thethyratron. Since the circuit formed by these parts causes only smallenergy losses, discharge occurs in the form of a slightly dampedvibration, however, only a half-period is passed through, as thethyratron permits flux only in one direction. But for elucidation, thedotted line in Fig. 3 shows some periods of the slightly dampedvibration. (In most cases its frequency is much higher inv comparison tothe network frequency than the figure indicates, for reasons ofdrafting.) Hence, shortly after the passage through zero of the networkvoltage, the voltage U; at the condenser 4 remains at a negative value,which is almost as great as the productive value prevailing before theexpansion. The recharging process of the condenser 4 brings about theoccurrence of a current impulse in coil 2 which removes the electronsoutward from their circular path. The said negative value of voltage U,is now reached by voltage U shortly before its negative peak. As thethyratron 6 is still unlockedthe times of unlocking of thyratrons 6 and7 are marked by thick lines on the axes 6 and 7 respectively of Fig.3condenser 4 is then charged to the negative peak of voltage U Thenthyratron 6 is locked again.

During the next passage through zero of the network voltage therefollows the expansion for the electrons accelerated in the otherdirection, in that by ignition of the thyratron 7 the slightly dampedvibration circuit consisting of condenser 4 and coil 2 again carries outa free vibration of the duration of a half period.

The described processes repeat periodically. The current impulsesoccurring in coil 2 are entered along the axis i of Fig. 3. Fig. 4 showsthe processes occurring during contraction. it is presupposed,therefore, that the reversing switch 9 (Fig. l) is in position k andthat at the same time the phase position of the positive grid voltageimpulses supplied to the thyratrons 6 and 7 has been changed by 180 bythe reversing switch 10. Compare lines 6 and 7 of Fig. 4 with thecorresponding lines of Fig. 3. Curve U of Fig. 4 represents the courseof the voltage connected to the series connection of the thyratron andcondenser 4, again referred to point c of Fig. 1. According to theinvention, voltage U is obtained, in that the exciter winding 1 and thecoil 2 are connected in series in phase opposition. In order that thevoltage U effective on contraction, will be of the ame magnitude as thevoltage U effective during expansion, coil 2 should be so dimensionedthat the alternating voltage induced in it by the acceleration flux hashalf the magnitude of the network voltage. If it should be desirablethat the impulses for contraction and expansion differ in amplitude,this can be taken into account by a variation of the proportions of coil2. Curve U of Fig. 4 shows the course of the voltage connected at thecondenser 4, likewise referred to point c of Fig. 1. The processes areanalogous to those described above in connection with expansion.However, the discharge of condenser 4 upon ignition of a thyratronproceeds not only via coil 2 but via the parallel connection of theexciter winding 1 and condenser with which it is in series. Thiscondenser, however, has such a low impedance for the impulse currentsthat the mentioned parallel connection has no disadvantageous effect onthe contraction process. It is seen by comparison of Figs. 4 and 3 withFig. 2 that the direction of the current impulses (i flowing in coil 2is reversed in relation to the direction of movement of the electrons. Asimplification of the device, whereby in particular the reversing switch10 can be locked in, can be achieved in that the feed voltage for thearrangement 8 is taken, not from the network, but for expansion, fromcoil 2, and for contraction, from the series connection of coil 2 withthe exciter winding 1. For this purpose, lines f, g of arrangement 8should be connected with points c, d of the connection. If necessary, alowpass filter may be arranged in front of arrangement 8, preventing theimpulse-like currents flowing in coil 2 from reaching the entrance ofarrangement 8.

Although the device of the invention is suitable particularly for use indua-radiation accelerators, it is understood that it can be used also inan accelerator in which electrons are periodically accelerated in onlyone direction of movement.

I claim:

1. The combination in a device for the selective generation of expansionor contraction impulses to effect expansion or contraction of theelectron orbit in a magnetic induction type electron accelerator Whoseaccelerzu tion flux is produced by an exciter winding fed from analternating supply voltage, of a coil embracing the acceleration fluxproduced by said exciter winding, :1 first terminal end of said coilbeing connected to the co phased first terminal end of said exciterwinding, two thyratrons connected in parallel in back-to-front relation,means for alternately applying voltage impulses to the control grids ofsaid thyratrons for rendering said thyratrons conductive in alternationeach time the supply voltage passes through zero, a condenser connectedat one terminal thereof with one common terminal of said thyratrons, aconnection between the other terminal of said condenser and the oppositeterminal end of said coil, and circuit means including switching meansmovable to alternative positions for selectively connecting saidthyratrons and condenser to said exciter winding and coil, saidswitching means in one position thereof connecting the other commonterminal of said thyratrons to said first terminal end of said coilthereby to connect said thyratrons and condenser only to said coil, andsaid switching means in the other position thereof connecting the othercommon terminal of said thyratrons to the other terminal end of saidexciter winding thereby connecting said exciter winding and coil inseries and wherein the respective voltages on said exciter winding andcoil are opposed.

2. A device for selective expansion or contraction of the electron orbitin a magnetic induction accelerator according to claim 1, characterizedin that said coil is so dimensioned that the alternating voltage inducedin it by the acceleration flux during operation of the acceleration isat least approximately equal to half the terminal voltage of saidexciter winding.

3. A device for selective expansion or contraction of the electron orbitin a magnetic induction accelerator as defined in claim 1 wherein saidvoltage impulses applied to the control grids of said thyratrons arederived from said supply voltage and which further includes secondswitching means operated simultaneously with said switching means andconnected between said supply voltage and said means for applyingvoltage impulses to the control grids of said thyratrons for changingthe phases of said voltage impulses by 4. A device for selectiveexpansion or contraction of the electron orbit in a magnetic inductionaccelerator as defined in claim 1 wherein said means for applyingvoltage impulses to said control grids of said thyratrons includes avoltage source established across said series connected thyratrons andcondenser.

5. A device for selective expansion. or contraction of the electronorbit in a magnetic induction accelerator as defined in claim 4 andwhich further includes a low-pass filter connected between the voltagesource established across said series connected thyratrons and condenserand said means for applying voltage impulses to said control grids ofsaid thyratrons.

References Cited in the file of this patent UNITED STATES PATENTS Re.23,217 Snyder Apr. 4, 1950 2,207,392 Zuschlag July 9, 1940 2,480,169Westendorp Aug. 30, 1949 2,654,838 Wideroe Oct. 6, 1953 2,754,419Wideroe July 10, 1956

